A typical plasma etching apparatus comprises a reactor in which there is a chamber through which reactive gas or gases flow. Within the chamber, the gases are ionized into a plasma, typically by radio frequency energy. The highly reactive ions of the plasma gas are able to react with material, such as a polymer mask on a surface of a semiconductor wafer being processed into Integrated Circuits (IC's ). Prior to etching, the wafer is placed in the chamber and held in proper position by a chuck or holder which exposes a top surface of the wafer to the plasma gas. There are several types of chucks known in the art. The chuck provides an isothermal surface and serves as a heat sink for the wafer. In one type, a semiconductor wafer is held in place for etching by mechanical clamping means. In another type of chuck, a semiconductor wafer is held in place by electrostatic force generated by an electric field between the chuck and wafer. The present invention is applicable to both types of chucks.
In semiconductor processing, the etch or deposition rate uniformity across the wafer during each process directly affects the device yield. This has become one of the main qualifying requirements for a process reactor and hence is considered a very important parameter during its design and development. With each increase in the size of wafer diameter, the problem of ensuring uniformity of each batch of ICs from larger and larger wafers becomes more difficult. For instance, with the increase from 200 mm to 300 mm in wafer size and smaller size per wafer, the edge exclusion shrinks to, for example, 2 mm. Thus maintaining uniform etch rate, profile, and critical dimensions all the way up to 2 mm from the edge of the wafer has become very important.
In a plasma etch reactor, the uniformity of etch parameters' (etch rate, profile, CD, etc.) is affected by several parameters. Maintaining uniform plasma discharge and hence plasma chemistry above the wafer has become very critical to improve the uniformity. Many attempts have been conceived to improve the uniformity of the wafer by manipulating the gas flow injection through the showerhead, modifying the design of the showerhead, and placing edge rings around the wafer.
The problem in a capacitively-coupled etching reactor having electrodes of different sizes is the lack of uniform RF coupling especially around the edge of a wafer. FIG. 1 illustrates a conventional capacitively-coupled plasma processing chamber 100, representing an exemplary plasma processing chamber of the types typically employed to etch a substrate. Referring now to FIG. 1, a chuck 102, representing the workpiece holder on which a substrate, such as a wafer 104, is positioned during etching. The chuck 102 may be implemented by any suitable chucking technique, e.g., electrostatic, mechanical, clamping, vacuum, or the like. During etching, the chuck 102 is typically supplied with dual RF frequencies (a low frequency and high frequency), for example 2 Mhz and 27 Mhz, simultaneously during etching by a dual frequency source 106.
An upper electrode 108 is located above the wafer 104. The upper electrode 108 is grounded. FIG. 1 illustrates an etching reactor where the surface of the upper electrode 108 is larger than the surface of the chuck 102 and the wafer 104. During etching, plasma 110 is formed from etchant source gas supplied via a gas line 112 and pumped out through an exhaust line 114. An electrical insulator ring 109 insulates the upper electrode 108 from the grounded chamber 100.
Confinement rings 116 may be placed between the upper electrode 108 and a bottom electrode, such as the chuck 102 in FIG. 1. In general, confinement rings 116 help confine the etching plasma 110 to the region above the wafer 104 to improve process control and to ensure repeatability.
When RF power is supplied to chuck 102 from RF power source 106, equipotential field lines are set up over wafer 104. The equipotential field lines are the electric field lines across the plasma sheath that is between wafer 104 and the plasma 110. During plasma processing, the positive ions accelerate across the equipotential field lines to impinge on the surface of wafer 104, thereby providing the desired etch effect, such as improving etch directionality. Due to the geometry of the upper electrode 108 and the chuck 102, the field lines may not be uniform across the wafer surface and may vary significantly at the edge of the wafer 104. Accordingly, a focus ring 118 is typically provided to improve process uniformity across the entire wafer surface. With reference to FIG. 1, wafer 104 is shown disposed within a focus ring 118, which may be formed of a suitable dielectric material such as ceramic, quartz, plastic, or the like. Thus the presence of the focus ring 118 allows the equipotential field lines to be disposed substantially uniformly over the entire surface of the wafer 104.
An electrically conductive shield 120 substantially encircles the focus ring 118. The electrically conductive shield 120 is configured to be substantially grounded within the plasma processing chamber. The shield 120 prevents the presence of unwanted equipotential field lines outside of focus ring 118.
Because the upper electrode 108 is larger than the bottom electrode 104, the path traveled by the RF current between the wafer 104 and the top electrode 108 increases especially at the edge of the wafer 104. Thus, the etch rate on the wafer 104 drops at the outer edge of the wafer 104 resulting in a less uniformly etched wafer.
Accordingly, a need exists for a method and apparatus for improving the plasma discharge uniformity above the wafer. A primary purpose of the present invention is to solve these needs and provide further, related advantages.